For lines that start prior to the official start of the party, CMs will come through the line checking for bands. Anyone without one will be kicked out of the line. I've seen it happen
Once the party starts, CMs will be stationed at the entrance to each land and will ask to see your band as you enter.
If you don't try to enter any lands or ride any rides, I guess it's possible you could slip through the cracks. I've never seen CMs actively patrolling the streets looking for bands except prior to the first parade.